The present invention relates to a spark plug and an ignition apparatus which are provided for an internal combustion engine of an automotive vehicle to ignite the fuel mixture introduced into a combustion chamber.
Conventionally known for improving the combustibility of fuel mixture is, for example, a multi-ignition technique using a plurality of spark plugs provided in the same combustion chamber or an intake air amount increasing technique using an intake port having an enlarged diameter.
According to these combustibility improving techniques, a very limited space is available for a spark plug. Under such circumstances, downsizing of spark plug as well as downsizing of ignition coil are keys to effectively utilize a limited combustion space.
An effective method for realizing the downsizing of spark plug and ignition coil is to reduce a discharge gap, i.e., a spatial clearance between a center electrode and a ground electrode disposed in an opposed relationship. This method is effective in reducing a discharge voltage, i.e., a voltage required for igniting fuel mixture.
However, according to the research and development of inventors of this invention, simply narrowing a discharge gap will result in worse ignitability of fuel mixture because the electrodes tend to obstruct the growth of a flame kernel caused in a narrowed discharge gap. In other words, the electrodes absorb the heat of a flame before the flame kernel grows sufficiently.